Firstly, each project should be designed with standards and learning objectives in mind: what does the project aim to achieve? what knowledge, information, or skills are the students expected to learn and master? How are their achievements expected to be assessed?
Therefore, both formative and summative assessments should be carefully designed before implementing the project. It's important to plan for peer assessment, self assessment (surveys and reflective writing), as well as teacher assessment - tests, quizzes, writing products, oral presentations, etc.
More on the assessment for PBL can be found here: Assessment.
Source: Our class materials.